144 VETERINARY PHYSIOLOGY 



(1) One eye (monocular vision). 



A. The method in which rays of Hght are 



focussed (dioptric mechanism). 



B. The stimulation of the retina. 



(2) Two eyes (binocular vision). 



2. The conduction of the nerve impulses from the retina to 

 the brain. 



8. The mode of action of the parts of the brain in which 

 the changes accompanying visual sensations are set up 

 (the visual centre). 



1. THE MODE OF FORMATION OF PICTURES UPON 



THE RETINA. 



(I.) Monocular Vision. 

 7. The Dioptric Mechanism. 



1. Distant Vision. — The eye may be compared to a photo- 

 graphic camera, having in front a lens, or lenses, to focus 

 the light upon the sensitive screen behind. The picture 

 is formed on the screen by the luminous rays from each 

 point outside being concentrated to a point upon the screen. 

 This is brought about by refraction of light as it passes through 

 the various media of the eye — the cornea, aqueous, crystalline 

 lens, and vitreous. The refractive indices of these, compared 

 with air as unity, may be expressed as follows : — 



Cornea . .1-33 Lens . .1-45 



Aqueous . 133 Vitreous . 1"33 



When a ray of light passes from a medium of lower into 

 one of higher refractive index, it is bent towards a line 

 drawn at right angles to the surface (fig. 65), and, conversely, 

 in passing from a medium of higher into one of lower re- 

 fractive index, it is bent away from a line at right angles to 

 the surface. 



Light therefore passes from a medium of one refractive 

 index into a medium of another refractive index (fig. 65) — 

 1. At the anterior surface of the cornea, from a medium 



